Cluster Ion Spectrometry Experiment Research Articles

Evidence of strong energetic ion acceleration in the near-Earth magnetotail

Until now it is still questionable whether ions are accelerated to energies above 100 keV in the near-Earth current sheet (CS), in the vicinity of a possible near-Earth neutral line. By using 11 years of 3-D energetic ion flux data for protons, helium, and oxygen (~150 keV–1 MeV) from the RAPID instrument on board Cluster 4, we statistically study the energetic ion acceleration by investigating ion anisotropies in the near-Earth magnetotail (−20 RE < X <−16 RE). It is found that the earthward (tailward) anisotropy of the energetic (>150 keV) ions (protons, He+, and O+) tend to become higher as the earthward (tailward) plasma bulk flows (measured by Cluster Ion Spectrometry experiment) become stronger. During such periods the presence of a strong acceleration source tailward (earthward) of Cluster spacecraft (S/C) is confirmed by the hardening energy spectra of the earthward (tailward) energetic ion flows. A good statistical correlation between tailward bulk flow, negative Bz, and the tailward anisotropy of energetic ions indicates that the strong ion acceleration might be related to a near-Earth reconnection, which occurred earthward of the Cluster S/C. The energetic ion anisotropies do not show a clear dependence on the AE index, which may indicate that the acceleration source(s) for the energetic ions could be spatially localized.

In-flight calibration of the Hot Ion Analyser on board Cluster

Abstract. The Hot Ion Analyser (HIA), part of the Cluster Ion Spectrometry experiment, has the objective to measure the three-dimensional velocity distributions of ions. Due to a variety of factors (exposure to radiation, detector fatigue and aging, changes in the operating parameters, etc.), the particles' detection efficiency changes over time, prompting for continuous in-flight calibration. This is achieved by comparing the HIA data with the data provided by the WHISPER (Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation) experiment on magnetosheath intervals, for the high-sensitivity section of the instrument, or solar wind intervals, for the low-sensitivity section. The paper presents in detail the in-flight calibration methodology, reports on the work carried out for calibrating HIA and discusses plans to extend this activity in order to ensure the instrument's highest data accuracy.

On the origin of field-aligned beams at the quasi-perpendicular bow shock: multi-spacecraft observations by Cluster

Abstract. Two distinct populations of reflected and accelerated ions are known to originate from quasi-perpendicular shocks, gyrating ions and reflected ion beams. Recent observations under such bow shock conditions with Cluster have shown strong evidence that both particle distributions appear to emerge from the same reflection process. In this paper the basic production mechanism of field-aligned beams has been investigated by using CLUSTER multi-spacecraft measurements. We have analyzed several quasi-perpendicular shocks with the Cluster Ion Spectrometry experiment (CIS) and followed the spatial and temporal evolution of the reflected and transmitted ion populations across the shock. These observations show that the field-aligned beams most likely result from effective scattering in pitch angle during reflection in the shock ramp. Investigating a low Mach number shock, leakage of a fraction of the thermalized ion distribution in the downstream region does not appear to be the source as the volume in phase space occupied by beam ions is empty downstream of the shock ramp.

Production of gyrating ions from nonlinear wave–particle interaction upstream from the Earth's bow shock: A case study from Cluster-CIS

We present observations of E<40 keV/e backstreaming ions measured by the Cluster Ion Spectrometry experiment at 1–2 R E upstream of the bow shock. The ions are simultaneously observed at all three spacecrafts for which CIS measurements are available. The proton distributions are analyzed using 4 s time resolution. They are observed in association with low-frequency quasi-monochromatic waves with substantial amplitudes (δ B /B∼1) . When the waves are present the ion distributions appear as gyrophase-bunched gyrating distributions while field-aligned beams are also observed just adjacent to the interval of wave occurrence. The gyrating distributions are observed at distances larger than an ion Larmor radius and they have pitch-angles inconsistent with a specular reflection mechanism at the bow shock. The properties of the ULF waves reveal that they are in cyclotron resonance with the ion parallel beams that could drive a right-hand ion/ion instability responsible for the wave occurrence. Moreover, the observed pitch-angles for the gyrating ion distributions are consistent with the theoretical value expected if they are produced by a coherent nonlinear wave–particle interaction.

Observations of the spatial and temporal structure of field-aligned beam and gyrating ring distributions at the quasi-perpendicular bow shock with Cluster CIS

Abstract. During the early orbit phase, the Cluster spacecraft have repeatedly crossed the perpendicular Earth’s bow shock and provided the first multi-spacecraft measurements. We have analyzed data from the Cluster Ion Spectrometry experiment (CIS), which observes the 3D-ion distribution function of the major species in the energy range of 5 eV to 40 keV with a 4 s resolution. Beams of reflected ions were observed simultaneously at all spacecraft locations and could be tracked from upstream to the shock itself. They were found to originate from the same distribution of ions that constitutes the reflected gyrating ions, which form a ring distribution in the velocity space immediately upstream and downstream of the shock. This observation suggests a common origin of ring and beam populations at quasi-perpendicular shocks in the form of specular reflection and immediate pitch angle scattering. Generally, the spatial evolution across the shock is very similar on all spacecraft, but phased in time according to their relative location. However, a distinct temporal structure of the ion fluxes in the field-aligned beam is observed that varies simultaneously on all spacecraft. This is likely to reflect the variations in the reflection and scattering efficiencies.Key words. Interplanetary physics (planetary bow shocks; energetic particles; instruments and techniques)